Conventionally, there is known a plasma processing apparatus which includes a flat antenna, as shown in FIG. 18.
This plasma processing apparatus 71 comprises a processing container 73 generally shaped to be cylindrical with a bottom and a silica plate 75 formed on the ceiling part of the processing container 73 in an airtight manner thereby to define a closed processing space S in the processing container 73. Accommodated in the processing container 73 is a mounting table 77 on which a semiconductor wafer W is mounted. This mounting table 77 is connected to a bias high-frequency power source 79 through power lines. Further, a gas nozzle 81 is arranged in the sidewall of the processing container 73, for introducing a process gas into the container. The processing container 73 is also provided, on a bottom thereof, with exhaust ports 85 connected with a not-shown vacuum pump.
On the other hand, a flat antenna member 87 is arranged on the top of the silica plate 75 sealing up the upside of the processing container 73. The flat antenna member 87 is constituted as a bottom plate of a radial waveguide box 89 consisting of a low and disc-shaped hollow cylindrical container. The flat antenna member 87 is attached to a top surface of the silica plate 75. A coaxial waveguide 93 has its outer tube 93A connected to the center of an upper face of the disc-shaped radial waveguide box 89. The coaxial waveguide 93 is also connected, at the other end, with a microwave generator 91. In the coaxial waveguide 93, an inside cable 93B is connected to the center of the disc-shaped antenna member 87.
The disc-shaped antenna member 87 is made from a copper plate having a number of slits 95 formed therein. Further, in the radial waveguide box 89, a dielectric material 97 of predetermined dielectric constant is accommodated to shorten the wavelength of a microwave thereby accomplishing a short guide wavelength.
With the above structure, a microwave generated in the microwave generator 91 is propagated in the coaxial waveguide 93 and successively dispersed in the radial waveguide box 89 in the radial direction. Then, the microwave is discharged downward from the slits 95 of the antenna member 87 thereby to form a plasma in the processing container 73.
However, since cables inside the coaxial waveguide are easy to be heated in the above processing apparatus 71, such an overheating operation may cause an abnormal discharging of electricity in the apparatus. In order to prevent the occurrence of abnormal discharging, it is necessary to provide the “so-slender” inside cable with a cooling mechanism. However, this countermeasure would cause the structure of the apparatus to be complicated with an excessive increase in manufacturing cost. Additionally, since the countermeasure requires a supporting structure for the inside cable, a new problem arises in that it might take a great deal of time to adjust an impedance accompanied by the provision of the supporting structure.
Further, due to the generation of uneven electric field formed below the flat antenna member 87, the processing apparatus 71 has a problem of producing an uneven treatment on the wafer W. In detail, an electric field emitted downward from the slits 95 of the flat antenna member 87 is reflected on an inner wall of the processing container 73 to produce an uneven electric field in the processing container. Thus, the above processing apparatus betrays an uneven treatment in processing wafers, especially, large-diameter wafers.
In order to solve the above-mentioned problems, the object of the present invention is to provide a plasma processing apparatus which is capable of prevention of heat-generation of the cable inside the coaxial waveguide and which can form a uniform electromagnetic field in the processing container.